Autism spectrum disorders (ASD) have common core symptoms which include increases in anxiety, and repetitive behaviors, with reduced sociability. It is widely believed that ASDs arise from subtle differences in the equilibrium between excitatory and inhibitory GABAergic neurotransmission. The electroneutral K+/Cl- co- transporter 2 (KCC2, or SLC12A5) is selectively expressed in the CNS after birth and allows neurons to maintain low intracellular Cl- concentrations an essential pre-requisite for the postnatal development of inhibitory neurotransmission. Consistent with its role in facilitating neuronal inhibition, deficits in KCC2 expression are evident in patients with ASDs and multiple autism animal models. KCC2 activity is subject to both positive and negative modulation via phosphorylation of serine's 940 and threonine residues 906 and 1007 respectively. Here, we will directly test if these regulatory processes influence the postnatal development of GABAergic inhibition, and the pathophysiology of ASDs. To do so we have created mice in which positive modulation of KCC2 activity by phosphorylation has been ablated via mutation of S940 to an alanine (S940A). T906/1007 are phosphorylated by with with-no-lysine kinases (WNKs), and we have also obtained line mouse lines with deficits in WNK activity. Preliminary studies using these new reagents have allowed us to formulate a novel hypothesis that will be tested here: The postnatal activation of KCC2 is facilitated by the reciprocal phosphorylation of S940 and dephosphorylation of T906/1007. Compromising these processes decreases the efficacy of GABAergic and directly contributes to the pathophysiology of ASDs. The experiments we will perform to test our hypothesis are detailed in the following aims. Specific Aim 1. To test the hypothesis that phosphorylation of S940 in KCC2 is a critical determinant for the postnatal development of GABAergic inhibition. Specific Aim 2. To test the hypothesis that WNK dependent phosphorylation of KCC2 slows the postnatal development of hyperpolarizing GABAergic inhibition. Specific Aim 3. To test the hypothesis that slowing the postnatal development of GABAergic inhibition reproduces the core behavioral deficits of ASDs. Collectively, these experiments will provide key mechanistic insights into how deficits in KCC2 phospho-dependent modulation contribute to the pathophysiology of ASD. This information may lead to the development of more effective therapies targeting KCC2 activity to ultimately improve patient outcomes for ASDs. Such strategies may also be relevant for other neuropsychiatric disorders, such as epilepsy, neuropathic pain, and schizophrenia in which deficits of KCC2 activity are believed to be of significance.